Why does Macbeth kill King Duncan?  

Lady Macbeth forcefully encourages Macbeth to murder Duncan, and eventually he gives over to her appeal and his own desire for power, which increases steadily after the three witches tell him that he will be king. The question posed to Macbeth by the witches' prophecy is whether to let the prophecy come true on its own or to take fate into his own hands, which he does.

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Macbeth kills Duncan because of his "vaulting ambition" to become King of Scotland and his desire to please his domineering wife, who plots Duncan's murder and assists him in executing the bloody crime. Shortly after the witches greet Macbeth in act 1 by calling him Thane of Glamis, Thane of Cawdor, and future King of Scotland, Ross and Angus arrive and inform Macbeth that he has been given the title Thane of Cawdor, which coincides with the witches' prophecy. This news incites Macbeth's ambition, and he immediately entertains the idea of assassinating Duncan to become king. After Macbeth informs his wife of the favorable prophecies, she begins plotting Duncan's assassination.

Even though Macbeth desires to become king, his conscience initially prevents him from committing the crime and he decides to remain a loyal, trustworthy subject. During his soliloquy in Act 1, Scene 7, Macbeth admits that his "vaulting ambition" is his only motivating factor. Despite Macbeth's reluctance to obey his ambition, Lady Macbeth manipulates him by questioning his masculinity and assuring him of their success. After speaking with his cruel wife, Macbeth succumbs to his ambition and commits regicide by stabbing Duncan in his sleep. Following the assassination, Macbeth develops into a heartless, bloodthirsty tyrant, who continues to be manipulated by the witches and is eventually killed by Macduff during the final battle.

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Macbeth first thinks about killing King Duncan after the the three witches prophesize that he will one day be king himself. After he hears this, he asks himself why he contemplates "that suggestion / Whose horrid image doth unfix my hair / And make my seated heart knock at my ribs, / Against the use of nature?" The "suggestion" he is tempted by is the idea that he should kill Duncan, which, as he says, is a sinful thought to have, and unnatural, or "Against the use of nature." He subsequently says in act 1, scene 3, that, "If chance will have me king, why chance may crown me, Without my stir." In other words, if it is his fate to be king, then he should simply let fate take its course and does not need to "stir," or help it along. This implies firstly that he was previously thinking about intervening and helping fate along by killing the king, and secondly that he has now decided that he will not kill the king, but let fate unwind as it will.

Macbeth does not find it easy to simply sit back and let fate take its course, without his help. He is, after all, a proud man of action, used to determining his own fate. In act 1, scene 5, he implores the heavens to "hide (its) fires" and "Let not light see (his) black and deep desires." Here then we can see that Macbeth still has strong and "dark desires" to kill the king, but we can also see that he tries to fight against those desires.

Macbeth's subsequent decision to succumb to his "dark desires" and kill the king can be largely credited to his wife. She decides that when King Duncan comes to their home the next day, he will not leave alive. However, in act 1, scene 7, Macbath acknowledges that he has only one reason to kill Duncan, and that is to satisfy his own ambitions. Indeed, he says that he has no motivation, or "spur / To prick the sides of (his) intent, but only / Vaulting ambition, which o'erleaps itself." Realizing this, he tells his wife that they "shall proceed no further in this business," meaning that they will not kill Duncan. Lady Macbeth nonetheless persists and accuses Macbeth of cowardice, asking him if he is going to "live a coward in (his) own esteem," and telling him to "screw (his) courage to the sticking-place." This proves to be all the persuasion Macbeth needs. He decides to kill Duncan after all and tells his wife that he is now "settled" upon that course.

In summary then, Macbeth's decision to kill King Duncan has two main causes. The first is his own "vaulting ambition," which first gives rise to his "dark desires." The second is his determination always to be the courageous man of action, which his wife exploits. His own ambition is the driving force behind the decision, but he needs his wife to give it a nudge in the right direction. In this way, his ambition is like a boulder teetering on the edge of a cliff, and his wife is the force which tips the boulder over the edge and starts the avalanche.

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There are several reasons for his decision. Macbeth has been told by the witches that he will eventually become king. If Duncan is already king, how can Macbeth become king? Answer: kill Duncan. However, this is Macbeth's first mistake. The witches operate on the basis of fate--it is destined for Macbeth to become king. However, Macbeth takes matters into his own hands (rather than just letting events play out as they will), and that disrupts fate's system. (Or you can argue that it was fated that Macbeth would kill Duncan and that's what the witches were predicting when they said he's become king.)

Lady Macbeth also encourages Macbeth to kill Duncan. She is almost more status and power hungry than he is. She calls his manhood into questions, saying if he was a true man, he would do this. She uses all sorts of feminine manipulation to persuade him into killing Duncan--and Macbeth doesn't want to see like less of a man to his wife.

When the deed is actually to be committed, Lady Macbeth said she would have killed Duncan, but he looked too much like her father when he was asleep. Therefore, it leaves Macbeth to actually do the killing.

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